1. Field of the Invention
The field of the invention particularly is that of display screens and more generally that of devices using composite materials capable of passing from a transparent or scattering state to a scattering or transparent state respectively under the effect of an electrical or optical command.
In the case of display screens made with polymer/liquid crystal components capable of containing dichroic dyes, the effect brought into play is an electro-optical effect: generally, a film of a composite material formed by polymer and dyed liquid crystal is contained between two transparent conductive electrodes. The dyed liquid crystal is obtained by the dilution of dyeing molecules between liquid crystal molecules. At rest, without voltage, the film is a scattering film owing to the differences in refractive index between the polymer and liquid crystal and is furthermore absorbent because of the molecules of dye whose orientation, dictated by that of the liquid crystal, varies randomly from one domain to another. The application of a voltage to the terminals of this film causes the molecules of liquid crystal to get oriented with the molecules of dye chosen so as to be dichroic, in parallel to the electrical field applied. By working firstly with a liquid crystal whose ordinary index is equal to that of the polymer (there is no scattering at the interface between the polymer and the liquid crystal) and secondly with a dichroic dye that is no longer absorbent, a transparent film is obtained under an electrical field. Such a film is therefore capable of providing two states, a scattering and absorbent state and a non-absorbent, transparent state. The contrast obtained with such composite materials is far more satisfactory than the one proposed with composites containing no dichroic dyes and having only one scattering state and one transparent state.
Furthermore, the use of composite material formed by polymer and dyed liquid crystal offers many advantages as compared with other display techniques using, in particular, liquid crystal cells. Indeed, these composites:
enjoy the advantage of ease of implementation of the polymers which makes it possible to deposit them easily in thin films having a controlled thickness on large surface areas; PA1 do not require any surface treatment of the substrates forming the screen; PA1 work without any polarizer (whence a substantial gain in luminosity); PA1 show an electro-optical effect with a viewing angle of the order of 150.degree.. PA1 the making of a porous film F of cross-linked polymer comprising liquid crystal (.chi..sup.L).sub.o within the pores; PA1 the immersion of said film in a solvent S of liquid crystal (.chi..sup.L).sub.o to dissolve said liquid crystal; PA1 the drying of said film under temperature and pressure conditions that are close to the supercritical phase or are in the supercritical phase; PA1 the filling of the dried film by molecules m.
2. Description of the Prior Art
Nevertheless, at the present time, the techniques for the implementation of these compounds are not satisfactory, especially because of remaining impurities.
Indeed, the main method used to obtain composite materials formed by liquid crystals and polymer is the method of phase separation induced by polymerization. This method consists in making a homogeneous mixture of a liquid crystal and a monomer and then polymerizing it (thermally or photochemically). The polymer that forms becomes insoluble in liquid crystal and separates therefrom.
Consequently, the fluid phase of the composite material contains chiefly liquid crystal but also monomer, fragments of initiator and trapped free radicals. On the one hand, these impurities reduce the resistivity of the material and therefore reduce the time constant of the capacitor formed by the composite material introduced between the two electrodes. On the other hand, the residual free radicals induce substantial ageing in the cell. It is therefore important to be able to purify the composite material thus obtained by withdrawing the fluid phase and then introducing a clean mixture of liquid crystal capable of containing dichroic dye.
Furthermore, the introduction of dyes cannot be done directly because they get deteriorated by the free radicals.
For these two reasons, it may be worthwhile to replace the liquid crystal used for making the composite material by a clean liquid crystal (capable of containing a dye): a first technique (K. Takenchi, Y. Umezu, H. Takatsu, 17th Japan LC Conference 108, September 1991) consists in opening the cell and washing the liquid crystal with a solvent.
It is also possible to bring about the diffusion of the solvent without opening the cell and then to evaporate this solvent.
The invention is based on the fact that the different approaches used to empty the polymer network affect this network whose porosity deteriorates during the elimination of the solvent. Indeed, when the liquid evaporates within the pores of the polymer, there is created a liquid/gas interface where forces due to the surface tension of the liquid are exerted. The sum of these forces has the effect of destroying the network. This deterioration then affects the properties of the liquid crystal/polymer composite material obtained after a new introduction of liquid crystal into the polymer network that has been washed beforehand.
The same type of problem may also arise in the "optical limiter" devices used to attenuate high power light beams capable of damaging optical systems. Indeed, to obtain the protection of detector type optical systems, it may be advantageous to integrate composite materials into these systems, these composite materials comprising a porous network of polymer in which there are included molecules whose refractive index may vary greatly under the effect of an intense light beam. By making a judicious choice of the molecules that initially have a refraction index equal to that of the polymer and a refraction index that is therefore appreciably different under high radiation, it is thus possible to go from a transparent state to a scattering state by optical control.
The molecules introduced may typically be of the CS.sub.2 type. Since these molecules have to be inserted into the polymer network, they could, in this respect, be formed by means of an initial liquid crystal (.chi..sup.L).sub.o as in the preparation of the composite material for a display screen.